In the field of the vehicle seat belt apparatus designed to protect a vehicle occupant in a vehicle seat, there has been used in recent years a technique for, in an emergency or during unstable travel of the vehicle, holding or restraining a vehicle occupant to a seat by means of the seat belt to thereby limit a posture change of the vehicle occupant. Such vehicle seat belt apparatus are provided with an electric-type pretensioner. Generally, the electric-type pretensioner includes a belt reel provided on a seat and having wound therearound a seat belt, a motor for rotationally driving the belt reel to take up the belt, and a control section that adjusts the amount of electric power supply (or electric current supply) to the motor to control a belt take-up driving force of the motor and thereby control the tension of the belt.
One example of the prior art vehicle seat belt apparatus of the aforementioned type is disclosed in Japanese Patent Application Laid-open Publication No. 2003-246257. In the vehicle seat belt apparatus disclosed in the No. 2003-246257 publication, where the seat belt tension is controlled by the motor, appropriate seat belt control is achievable even when a target value of the seat belt tension is varied to get smaller with the seat belt tension determined only by a spring force as a result of belt-paying-out control performed to cancel gear engagement. For that purpose, a motor drive control section of the vehicle seat belt apparatus disclosed in the No. 2003-246257 publication performs control such that the electric current supply to the motor is modified to a low target level or value so that, when the seat belt has been re-paid out, due to movement, posture change or the like of the vehicle occupant, a belt-paying-out electric current of such a very low level, which will not cause the seat belt to be further paid out substantively, is supplied to the motor for a predetermined time.
With the prior art vehicle seat belt apparatus where the driving of the motor is controlled by adjusting the amount of electric power supply (and hence electric current supply) to the motor so that the belt tension takes a target tension value, there is achieved the advantageous benefit that the belt tension can be stabilized. In a case where an electric current to be supplied to the motor is controlled in a “constant current mode”, the prior art vehicle seat belt apparatus would present the problem that, even when a constant electric current of an arithmetically-determined necessary value is supplied to the motor, a necessary driving force can not always be obtained due to a change in internal resistance value caused by an individual difference, aging, etc. of the motor. Namely, the belt tension obtained by the motor control would sometimes fail to take a predetermined value calculated by the control section. Particularly, in a case where a low electric current is supplied to the motor so that the vehicle occupant is restrained with an extremely small restraining force, or where slack of the seat belt is taken up gently, the performance or capability of obtaining a desired motor-driving force would greatly depend on the motor used.